The Open Shortest Path First (OSPF) routing protocol is an interior gateway protocol for routing in Internet Protocol (IP).
A domain using OSPF may be sub-divided into areas. An area is a logical grouping of OSPF routers and links that effectively divides an OSPF domain into sub-domains. Routers in an area maintain an identical link state data base, and flooding of link state advertisements (LSAs) is limited to an area thereby reducing the OSPF protocol's impact on CPU and memory and making possible the construction of hierarchical network topologies. Areas are identified by an area ID and a special area called the backbone has an area ID equal to zero. The backbone is connected to all other areas as a hub is connected to its spokes and all areas must communicate with each other across the backbone.
An internal router is a router having all interfaces belonging to the same area. An area border router (ABR) connects one or more areas to the backbone and acts as a gateway for inter-area traffic.
Each ABR may summarize or aggregate a number of sub-net or network address components into a single aggregate address which is inserted into the backbone using a summary LSA. For example, a number of contiguous component addresses in a first area can be summarized by a first ABR into a first aggregate address having a smaller address mask or prefix than the component addresses. Other ABRs will use the first aggregate address in their route tables to direct packets having destination addresses that match the aggregate address to the first ABR. The first ABR will match the destination address of each packet to the appropriate component address of the aggregate address using the larger sub-net address mask.
This address aggregation is a crucial requirement for scaling OSPF to large domains, as it results in significant reductions in routing table size, smaller link state data bases, and less network traffic to synchronize the router link-state data bases. On the other hand, address aggregation also implies loss of information about optimal paths to specific destinations, which can generally result in suboptimal routing.